The [All Layers] is a special folder
for sound designers. It contains all of the instrument
layers separately. The patches are “clean”,
without any filter effect and with default envelope settings.
They can be useful if you want to create new sounds from
scratch.

Please note that these categories are
not carved in stone, they're just our approach. For example
you can use a bass sound as lead, or a polysynth key as
monosynth lead or even a pad when you tweak it a bit...
it's really up to you!